Optical Properties Of Lead-Free Double Perovskite

Perovskites have become a promising candidate for photoelectronic applications due to many intrinsic virtues.However,lead toxicity and poor environmental stability seriously hindered its practical use.As a countermeasure,many double perovskite structure systems have been proposed due to their non-toxic constituent nature and enhanced stability.Amongst the various compositions with double perovskite structure,Cs₂Ag In Cl₆ has attracted more attention due to their direct bandgap,long carrier lifetime and easy processability(see Chapter 1 for details).In this thesis,two forms of lead-free double perovskite,including nanocrystal and single crystal,were successfully synthesized by designing different synthesis methods,and the energy transfer path was systematically studied.For lead-free double perovskite nanocrystals,the mainstream synthesis method was the hot-injection at high temperature which leads to a relatively low quantum yield of photoluminescence.This is mainly due to two reasons including,i)the quenching effect of metal silver,which is usually formed by Ag⁺reduction at high temperature in the presence of organic amines,and ii)the forbidden transition of pristine double perovskites.In Chapter 3,the synthesis of Cs₂Ag In Cl₆ nanocrystals at room temperature in an inverse microemulsion system was reported,where the reduction of Ag⁺is largely suppressed.By co-doping of Bi and Na ions,dark self-trapping excitons(STEs)were converted into bright ones,enabling a bright phosphor of photoluminescence quantum yield up to 56%.Importantly,the doping method at room temperature relaxed the spin forbidden transition of the Bi-6s² orbital(¹S₀?³P₂),revealing the fine structure of tri-band excitation profile.This spin regular relaxation was attributed to the symmetric destruction of the doped lattice and was also demonstrated by Raman spectroscopy.Bright nanocrystals were used as color-converting ink in a proof-of-concept experiment,which enabled a stable white-light LED to work in various environments,even under water,showing good application potential.In the case of lead-free double perovskite single crystals,in Chapter 4,single crystals were grown by a hydrothermal method.Through the co-doping of Na and Bi ions to synthesize the lead-free double perovskite single crystal with bright self-trapped yellow light.Upon the excitation of 325 nm laser,a multi-phonon peak superimposed on the self-trapped state photoluminescence was observed on the Raman fluorescence spectrum,which is considered to be the coupling of the self-trapped state and the vibrational energy level.The strong electron-phonon coupling,which plays a crucial role in the self-trapped exciton formation,is demonstrated by multiphonon non-radiative transitions and large Huang-Rhys factor.Multiphonon peaks superposed on a continuous band are due to the strong coupling of longitudinal-optical phonons to excited electronic states caused by tetragonally distorted Ag Cl₆octahedron.This phenomenon was first observed in a lead-free double perovskite single crystal,which helps to further understand the properties of the material.In Chapter 5,a new-type long afterglow lead-free double perovskite single crystal was grown by co-doping Na and Mn ions.The photoluminescence spectrum and the decay curve show that the single crystal has both self-trapped emission and red-light emission of Mn ions characteristic,and an energy transfer phenomenon was proven in between.Surprisingly,a long afterglow up to 5400 s was observed from the single crystal,which was attributed to shallow traps and deep traps based on thermal luminescence spectra.More importantly,compared to traditional long-lasting materials that need to be calcined at a high temperature(>900?),the hydrothermal synthesis at 180?was much cost-effective.The single crystal featuring high transparency or pale purple has high aesthetic value,which may find applications in the jewelry or decoration industry.